In conventional master-slave systems transferring data, a limited number of data transfer lines including a clock line, data lines, and chip selection lines are established between them to transfer synchronized serial data. Such conventional master-slave communication systems are widely used for inter-chip communication in many electronic apparatuses including cellular phones.
In this type of conventional communication system, slave devices must be held active at all time in order to wait for incoming signals. Therefore, the slave devices cannot be inactivated (sleep mode) even when they do not receive any data for a long time.
Hence, when used in electronic apparatus such as a cellular phone that utilizes batteries, conventional master-slave systems waste electric power during disconnected periods, shortening life of the batteries. This becomes serious when a multiplicity of slave devices are involved.
It is possible to save power during disconnected periods by providing additional signal lines between the master device and the slave device to inform the status of the slave device and control the slave device. However, in order to do this, the number of the signal lines must be increased, which inevitably increase the size, and hence the cost, of the system.